Every year, sepsis causes more deaths in U.S. hospitals than prostate cancer, breast cancer, and AIDS combined. Elderly patients are a particularly high-risk group, with an incidence rate of ~60% of all septic cases. This patient population is very vulnerable to the consequences of sepsis, showing 100-fold higher mortality than younger patients. Some of these deaths occur acutely after sepsis, but ~70% of these patients survive the initial infection, and succumb to opportunistic infections during the chronic phase of sepsis. The chronic stage of sepsis is important and is characterized by immunosuppression, but little is known about the mechanisms of sepsis-induced immunosuppression. CD4 T cells, essential for coordinating immune responses to opportunistic pathogens, are severely depleted during the acute stage of sepsis, and gradually recover throughout the immunosuppressive phase of sepsis. Our preliminary data indicates that certain Ag-specific CD4 T cell populations do not recover, despite quantitative restoration of total CD4 T cells. We suspect that the prolonged loss of Ag-specific CD4 T cells introduces gaps within the T cell repertoire. Thus, we will examine novel strategies aimed at enhancing CD4 T cell recovery and function during the immunosuppressive stage of sepsis. Cytokines, such as IL-2 and IL-7, show great promise in the treatment of sepsis immunosuppression, but they can be detrimental to septic patients because of non-specific, systemic toxicity. One way to minimize unintended toxicity while maximizing potency of a cytokine therapy is to use cytokine:?-cytokine mAb conjugates (cytokine complexes). The impact of IL-2 or IL-7 complexes in terms of CD4 T cell reconstitution, repertoire diversity, and pathogen clearance in sepsis survivors has not been thoroughly studied. Accordingly, our central hypothesis holds that sepsis-induced lymphopenia results in long-lasting changes in the composition and/or function of Ag-specific CD4 T cell populations, which ultimately are responsible for the reduced CD4 T cell response to pathogen-derived Ag encountered within the context of localized or systemic secondary infections. The following specific aims will test our hypothesis: Aim 1) Define the sepsis-induced intrinsic and extrinsic factors affecting the function of Ag-specific CD4 T cells; Aim 2) Investigate the abilityof cytokine complexes to improve CD4 T cell recovery and function after sepsis; and Aim 3) Evaluate the extent to which CD4 T cell recovery and function is controlled by commensal bacteria-derived Ag released during a septic episode. Ultimately, this application will increase our understanding of why septic patients are more susceptible to secondary infections. Our combined experience with the two-hit CLP sepsis model (CLP followed by a secondary heterologous infection) and peptide:MHC II tetramer approaches to study endogenous Ag-specific CD4 T cells positions us perfectly to accomplish the proposed experiments.